Which of the following was a multi-alphabet cipher widely used from the 16th century to the early 20th century?

The Vigenère cipher is the correct choice as it represents a significant advancement in cryptography due to its use of multiple alphabets throughout the encoding process. Unlike simpler ciphers, such as the Caesar cipher, which shifts letters uniformly, the Vigenère cipher employs a keyword to dictate the shifting of letters, using a different shift for each letter in the plaintext.

This multi-alphabet approach allows the Vigenère cipher to resist frequency analysis more effectively than single-alphabet ciphers, making it much more secure for its time. Each letter in the plaintext is combined with a corresponding letter from the keyword, which can repeat as necessary. This keying mechanism leads to a much more complex pattern that obscures the underlying message, significantly enhancing its cryptographic strength.

Though other ciphers such as the Caesar cipher, Atbash, and Scytale have their historical importance, they do not employ a multi-alphabet encryption scheme in the same way that the Vigenère cipher does. The Caesar cipher relies on a single shift across the entire message, Atbash is a monoalphabetic substitution cipher that merely reverses the alphabet, and the Scytale is a transposition cipher that has its own unique mechanics but does not utilize multiple alphabets for its encryption processes